1. Field of the Invention
The present invention pertains to apparatus for coating a substrate such a paper or plastic webbing.
2. Description of the Related Art
Two important techniques are used today in the art to apply a coating to a substrate. Each, however, has its own drawbacks. In one coating technique, a substrate is passed underneath a knife blade Positioned generally perpendicular to the path of travel of the substrate. A coating is applied with a greater thickness to the substrate, upstream of the knife blade. The material is typically much thicker than the desired final coating. The bottom, working edge of the knife blade is spaced a precise distance above the surface of the webbing to be coated, to meter the coating while setting a maximum thickness limit for the coating downstream of the knife blade. Also, the coating is spread onto the substrate surface by the knife blade, a feature which is sometimes relied upon to impart a desired finish to the coating. It is generally preferred that the coatings applied in this Manner are continuous and unchanging throughout the length of a production run, that is, from one section of substrate to another.
As those familiar with the art are aware, the knife blade application process offers significant advantages such as applying multiple coatings to a single substrate, but is subject to streaking which must be carefully monitored during a production run. The problems considered here are associated with small particles present in the coating material, which are of a size approaching the gap between the knife blade and the webbing surface. The knife gap in commercial applications is typically very small, of the order of 1 to 2 mils. The particles may comprise airborne contaminants, or perhaps paper fibers which are present in the environment. The particles may also comprise constituents of the coatings. Paint formulations typically include a liquid vehicle to which one or more colorants are added. These colorants often take the form of solid particles which are finely ground and dispersed throughout the paint base. Different colors and types of coatings have different coloring agents exhibiting a fairly wide variety of particulate sizes and characteristics. Some colors and coating types are especially prone to having larger size particles in the liquid suspension. Coatings containing these particles are applied to the substrate immediately upstream of the knife blade and are made to pass underneath the knife blade due to the momentum of the substrate. If the particles are of a size on the order of the gap between the knife blade and the substrate, an imperfection in the coating, which frequently is visible to the unaided eye as a streak, will result. In some applications, it is important that the coating be uniform throughout a relatively long production run. For example, in the manufacture of color samples, a substrate many feet in length will be coated with one or more stripes of different coating materials, and later divided into swatches or "chips" on the order of a inch square in size. Very often, a coating imperfection due to an overly large particle passing underneath a knife roller will be of a size sufficient to spoil several chips. While the coatings can be subjected to unusual preprocessing steps such as filtration or ultra-filtration techniques, these steps are of themselves costly to operation and may prove commercially impractical for some jobs.
In another popular technique used today, a series of rollers apply coating to a substrate. A primary roller is partly immersed in a coating material and transfers the material to a series of downstream rollers, which in turn, convey the material to a substrate. Roller coating devices can deliver a good quality coating across the width of a web, but cannot simultaneously apply multiple coatings to the same substrate, as can be done with the knife coating process.